The total protein content and the protein fractions were determined in the blood serum and the cerebrospinal fluid (CSF) of chick embryos and foetuses between the 4th and 20th day of incubation and of 2-day-old chickens (by Lowry's method and by polyacrylamide gel electrophoresis respectively). The main results were the following: The total protein content of the CSF increased from 1.8 mg/ml on day 4 to 3.1 mg/ml on day 10. After a slight decrease to 2.6 mg/ml on day 12 no further changes were detected. In the blood serum the total protein content was 4.3 mg/ml on day 4 and 4.7 mg/ml on day 9. Beginning with day 10 it increased constantly to 23.2 mg/ml on day 2 after hatching (daily rate 1-1.2 mg/ml). It results that the "protein barrier" between blood and CSF begins to function after 10-12 days of incubation. Of the protein fractions determined, the light fraction I in the CSF represents 50 per cent of the similar blood serum fraction (on day 10). As to the heavy fractions XI-XIV, their concentration in the CSF represents, beginning with day 10, 51 per cent of the similar blood serum fraction (until the end of the period investigated). It results that the "protein barrier" has an active, selective character. Some comparative aspects with literature data are discussed.